天文学：木星型の太陽系外惑星の形成過程が観測された

An image of a Jupiter-like exoplanet — called AB Aurigae b — forming at a long distance from its star (equivalent to 93 times the mean distance between Earth and our Sun) is presented in a paper in Nature Astronomy. The finding supports the idea that giant planets can be formed from large fragments of collapsing gas via gravitational instability, rather than only by the standard model used to explain Jupiter’s formation: core accretion.

The gas giant planets of the Solar System Jupiter and Saturn orbit 5 to 10 astronomical units (au) from the Sun — with one au equating to the mean distance between the Earth and the Sun. These planets were formed by the slow accumulation of smaller, rocky components known as planetesimals and the rapid accretion of gas onto this several Earths-mass core. However, a small minority of giant exoplanets (planets beyond the realm of our Solar System) have been imaged at distances of 50 to 200 au from their stars, where there are likely insufficient planetesimals to form them. These exoplanets are suggested to have instead been created via a process known as disc or gravitational instability.

Using the Subaru Telescope and the Hubble Space Telescope, Thayne Currie and colleagues observed a Jupiter-sized exoplanet, AB Aurigae b, forming within the gas disc around a young star called AB Aurigae. Spiral waves of disc material were also seen in the wake of AB Aurigae b — a phenomenon predicted by disc instability. The team estimates that the mass of AB Aurigae b is 9 times that of Jupiter. The authors suggest that multiple planets could orbit AB Aurigae, as indications of two candidate embryonic planets much further away — at radial distances of 430 and 580 au from AB Aurigae — were also observed.

The authors conclude that by providing a crucial first look into this early, embedded stage of planet formation, these findings could have important implications for our greater understanding of the evolution of massive gas giant planets.